Opentracing Utils (#5722)

* Add decerators for tracing functions

* Use the new clean contexts

* Context and edu utils

* Move opentracing setters

* Move whitelisting

* Sectioning comments

* Better args wrapper

* Docstrings

Co-Authored-By: Erik Johnston <erik@matrix.org>

* Remove unused methods.

* Don't use global

* One tracing decorator to rule them all.